A bit of background: the basis for this was David Sanders et al's paper (viewable here). In the British Election Study survey, they gave respondents an AV-style polling card and asked them how they would vote. They don't have enough respondents to estimate vote shares in every constituency, so instead they start with the First Past The Post vote shares from the real election, and estimate second and further preferences for each party's supporters using the BES survey data. By their reckoning, the Liberal Democrats would have picked up about 30 extra seats under AV -- not enough to make their seats proportional to their votes, but a big change, and enough to let them form a majority coalition with Labour. (Interesting what-if question: if Nick Clegg was part of a Labour coalition government, would he be listening to classical music and crying?)

When I read the paper, I realised that I could take this a step further, and use the survey data to estimate first preferences as well as second preferences. I reran the analysis. The results were not so good for the Lib Dems; instead of 30 seats, I estimate they would gain about 10.

Lab

Con

Lib Dem

Green/SNP/PC

Total

England

191

288

50

0

529

Scotland

40

1

12

6

59

Wales

27

7

5

1

40

Great Britain

258 (258)

296 (305)

67 (57)

0/6/1(1/6/3)

628 (630)

Estimated seats per party under the Alternative Vote, Great Britain

Notes: Numbers in brackets are seats in the actual election. Totalsare different because I'm missing data for a couple of constituencies.

It seems that AV would have made only a marginal difference to the election result. Of course, as both I and the original writers say, a simulation like this misses out lots of possible changes. Maybe people will vote differently once they are used to the AV system. Surely, by the next election, vote shares will be very different. Still, to the best of my judgment, AV is a small step rather than a massive revolution. I doubt that it will lead to many more coalitions, or to extremists getting elected, as some people claim.